FXO vs FXS: What's the Difference?

An FXS port supplies dial tone and ringing power, so you plug an analog phone or fax machine into it. An FXO port receives dial tone, so you plug an analog telephone line from the exchange (or a PBX extension) into it. In short: phones go into FXS, lines go into FXO.

Published 4 November 2026 · 3G Callnet

	FXS	FXO
Full name	Foreign Exchange Subscriber	Foreign Exchange Office
Supplies dial tone	Yes	No (receives it)
Plug in	Analog phone, fax, old PBX trunk	Telephone line from the exchange, PBX extension
Gateway use	Keep analog handsets/fax on a VoIP system	Bring analog lines into a VoIP system; PSTN failover

Typical setups

Office moving to an IP-PBX, keeping old phones → FXS gateway (one port per phone).
Keeping landlines with a new IP-PBX → FXO gateway (one port per line).
Hotels and hostels → high-density FXS gateways (24–112 ports).

Browse FXO/FXS gateways from Dinstar, Synway and Sangoma, or use the Product Finder.

Frequently asked questions

What does FXS stand for?

Foreign Exchange Subscriber — the port that serves a subscriber device such as a phone.

What does FXO stand for?

Foreign Exchange Office — the port that connects to the telephone office (exchange) line.

Can I connect FXO to FXO or FXS to FXS?

No. An FXS port must connect to an FXO device (a phone has an FXO interface), and an FXO port must connect to an FXS source (a phone line or PBX extension).